Nacl is a fcc lattice, where Na⁺ ions are at corner and face centre position. Chloride ions are at edge centres and body centre positions. How many Nacl formula units will be in an unit cell?

Nacl has fcc lattice structure , thus it has  ions  at corners and face center = 3 + 1 = 4  Na⁺ units
Cl^-  ions are edge center and at body center = 3 + 1 = 4 Cl^-  units 

Hence 4 units of Na⁺ and 4 units of Cl^-  ions give 4 units of Na⁺Cl^-